1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Description of the Related Art
There is generally employed a method of sealing a semiconductor chip with an epoxy resin or the like so that the semiconductor chip can be protected electrically, mechanically, or chemically from an outside. For example, the semiconductor chip is mounted onto a lead frame, and bonding pads provided on the semiconductor chip are connected to inner leads of the lead frame via bonding wires such as gold wires. After that, the semiconductor chip is sealed with the epoxy resin or the like for packaging.
The above-mentioned production method is suitable for mass production at low cost. For this reason, a full-mold package using the epoxy resin or the like is also increasingly applied to products (for example, high-power and high-frequency device), for ceramic packages are conventionally used.
However, in the case of using the epoxy resin, there was a problem in that moisture resistance deteriorates due to moisture absorption from inside the resin to the semiconductor chip. In addition, because a dielectric constant of the epoxy resin is as high as 3.9 to 4.5, there was another problem in that the parasitic capacitance (for example, gate-drain capacitance Cgd in a case of FET) was increased by the resin entering an element portion of the semiconductor chip, thereby deteriorating a high frequency characteristic.
As a technology for improving those defects, there is generally known a method involving covering and protecting a surface of the semiconductor chip with a polyimide-based resin, a silicon resin, fluorine-containing elastomer, a polymer resin, or the like.
FIG. 1 is a plan view illustrating an example of a semiconductor device according to the related art. FIG. 2 is a cross-sectional diagram taken along the line A of FIG. 1. A method of manufacturing the semiconductor device will be described. First, a semiconductor chip 101 is mounted on a lead frame 102, and is connected to inner leads of the lead frame 102 via bonding wires 103. After that, in order to improve the high frequency characteristic or protect the semiconductor chip 101, an upper portion of the semiconductor chip 101 is coated with a potting resin 105 made of fluorine-containing elastomer or a polymer resin which has a dielectric constant of as low as 2.0 to 2.4 and has high resistance to chemicals. Then, the semiconductor chip 101 is sealed for packaging with a package molding resin (sealing resin) 104 such as an epoxy resin.
As a result, the periphery of the element portion of the semiconductor chip 101 has a low dielectric constant, thereby reducing the parasitic capacitance between element electrodes and improving the high frequency characteristic. In addition, high resistance to chemicals can be obtained.
Further, JP 09-008181 A discloses a semiconductor device using a resin component whose molecules contain a carbodiimide unit (—R—N═C═N—; R is a divalent organic group) as a protective film of a semiconductor chip. The resin whose molecules contain the carbodiimide unit is dissolved in an organic solvent, which is used in a varnish state with a solution viscosity capable of application.
FIG. 3 is a cross-sectional diagram illustrating the semiconductor device disclosed in JP 09-008181 A. A silicon chip 201 is provided on a die pad of the lead frame 207 to be bonded with a gold-tin alloy, and an insulating film 202 made of silicon dioxide is formed on the surface of the silicon chip 201 into a thickness of about 100 nm. On apart of the insulating film 202, a bonding pad portion made of aluminum for drawing out an external electrode is formed and connected to a circuit. In the aluminum wiring layer, a passivation film 203 made of PSG, SiN, SiO2, or the like is formed into a thickness of from 50 nm to 200 nm.
A protective film 206 made of a resin component whose molecules contain a carbodiimide unit is formed so as to cover the passivation film 203. The bonding pad portion is electrically connected to the lead frame 207 via the bonding wires 205 such as a gold wire or an aluminum wire. The entire silicon pellet is sealed with a sealing resin 208 made of a thermosetting epoxy resin. By using the protective film 206, a water absorbing property or moisture permeability can be reduced, and the higher moisture resistance can be obtained as disclosed in JP 09-008181 A.
FIGS. 4 and 5 are cross-sectional diagrams illustrating other semiconductor devices disclosed in JP 09-008181 A. In the semiconductor device of FIG. 4, varnish is used in a post process. As illustrated in FIG. 5, the protective film 206 is formed on the whole of the surface of the semiconductor chip 201 and a back surface of the die pad. Also in those semiconductor devices, the same effects can be obtained as those of the above-mentioned semiconductor device of FIG. 3.
It should be noted that the related arts of the present invention include JP 2003-133512 A and JP 05-136312 A as well as JP 09-008181 A.
However, the potting resin 105 made of fluorine-containing elastomer or a polymer resin, which is used for the semiconductor devices shown in FIGS. 1 and 2, is a material used for improving the resistance to chemicals. Accordingly, the potting resin 105 has poor compatibility with the epoxy resin or the like which is the material of the sealing resin 104, and thus, contact property between the potting resin 105 and the sealing rein 104 is hardly obtained. For this reason, a void is generated in the interface between the potting resin 105 and the sealing resin 104, which raises such problems as deterioration in moisture resistance due to entrance of water from the outside, peeling, and cracking caused by a steam explosion due to thermal hysteresis at the time of packaging. Even in the case of using the resin component whose molecules contain a carbodiimide unit, which constitutes the protective film 206 as illustrated in FIGS. 3 to 5, the contact property with the semiconductor chip is improved, thereby improving the moisture resistance. However, the resin component has a similar problem in that the contact property with the sealing resin 208 is not yet fully improved while the contact property is better than that of the fluorine-based resin.
The problem can be prevented by, for example, reducing an application area of the potting resin 105 and by covering only the active region of the semiconductor chip 101 of FIGS. 1 and 2. This is because the interface between the sealing resin 104 and the resin 105 is made smaller. However, for example, an insufficient accuracy of a dropping position in a case of potting the resin, or fluctuation of a potting amount due to ununiformity of the resin viscosity causes fluctuation of coated shapes. As a result, the active region of the semiconductor chip 101 may not be covered in this case, and the sealing resin 104 having a higher dielectric constant enters the active region of the semiconductor chip 101, thereby increasing the parasitic capacitance and reducing the high frequency characteristic which were problems. This also applied to the semiconductor device shown in FIG. 3.
The problem of the fluctuation of the coated shape may be prevented by forming the protective film 206 on the whole of the surface of the semiconductor chip and the back surface of the die pad as in the structure illustrated in FIG. 5. However, by taking the structure, an area of the interface between the sealing resin 208 and the protective film 206 becomes larger, and the above-mentioned problem of the contact property arises. In the manufacturing process for the semiconductor device, in order to stably form the protective film on the entire surfaces as in the structure of FIG. 5, a device produced at high cost is required. Further, in such a package that requires to be grounded and to radiate heat by directly exposing the back surface of the lead frame as illustrated in FIG. 2, the protective film interferes with the contact between the back surface and the ground. For this reason, the structure of FIG. 5 cannot be applied.
On the other hand, as in the related art illustrated in FIG. 3, if the protective film 206 is formed in a preprocess before forming the semiconductor chip, the above-mentioned problems may be prevented. However, in this case, since the protective film 206 is always formed between the element electrodes, even in a case where the dielectric constant of the polycarbodiimide resin is to be further reduced, from a viewpoint of the improvement of the high frequency characteristic, it is difficult to obtain a state where the resin does not exist at all between the element electrodes, that is, the dielectric constant of the resin is not lowered to be equal to or lower than the dielectric constant of the air. Therefore, there is a limitation on the improvement of the high frequency characteristic from the viewpoint of increasing the parasitic capacitance component.